1. Field of the Invention
The present invention relates to a current injection-type optical semiconductor device and a semiconductor laser device in a frequency domain of a millimeter-wave band to a terahertz band (for example, from 30 GHz to 30 THz).
2. Description of the Related Art
A quantum cascade laser in which carrier transitions are performed between energy levels in the same energy band of either a conduction band or a valence band has been disclosed as a semiconductor laser. Since the oscillation wavelength of such a laser depends on the energy gap between two energy levels relating to optical transition, the oscillation wavelength can be selected over a wide spectrum range (for example, from a mid-infrared region to a terahertz band). In one example, laser light was achieved in a quantum cascade laser in which the oscillation wavelength was selected at 4.2 μm in the mid-infrared region. In another example, as disclosed by Rüdeger Köhler et al., (Nature, vol. 417 (2002), 156), a laser oscillation was confirmed even in a quantum cascade laser in which the oscillation wavelength was selected at 67 μm in the terahertz band. This suggests that such a semiconductor laser, in which carrier transitions are performed between energy levels in the same energy band of either a conduction band or a valence band, is useful as a light source in the terahertz band.
A description of a quantum cascade laser will now be provided with reference to FIG. 4.
FIG. 4 shows a part of the structure of a conduction band when an electric field is applied to a quantum cascade laser. A region A 410 includes potential barriers 441, 443, 445, and 447, and quantum wells 442, 444, and 446. This structure provides energy levels 411, 412, and 413 in the region A 410. A region B 420 includes potential barriers 447, 449, 451, 453, and 455, and quantum wells 448, 450, 452, and 454. This structure provides a mini-band 421 in which a plurality of energy levels is bundled. The region A 410 and the region B 420 are periodically repeated a plurality of times. A region A 430 indicates a region A in the subsequent period.
When a predetermined electric field is applied to the quantum cascade laser, an electric current flows. In particular, an electron undergoes an optical transition 401 from the energy level 411 to the energy level 412 in the region A 410, thereby emitting light having a wavelength corresponding to the energy gap between the energy level 411 and the energy level 412. Subsequently, the electron at the energy level 412 in the region A 410 is extracted to the region B 420 through the energy level 413 by, for example, optical phonon scattering 402. The electron passing through the mini-band 421 in the region B 420 is injected in the subsequent region A 430 and undergoes the optical transition as in the region A 410. Since the energy gap between the energy level 411 and the energy level 412 can be freely designed, light emitted by the quantum cascade laser can be selected over a wide spectrum range. Such light is resonated with an appropriate optical resonator, thereby performing laser oscillation.
In order that the oscillation wavelength is selected in the range from the millimeter-wave band to the terahertz band, the energy gap between two energy levels relating to the optical transition in the region A should be as small as the broadening of an energy level that is essentially inevitable (11 meV, K. K. Choi et al., Physical Review B, vol. 35 (1987), 4172) or less. Therefore, in the structure of the known quantum cascade laser in which the oscillation wavelength is selected in the range from the millimeter-wave band to the terahertz band, a non-radiative current path wherein carriers flow from the region A to the region B without undergoing the optical transition, or a non-radiative current path wherein carriers flow from the region B to the region A without undergoing the optical transition, forms a short-circuit. In a typical case, the carriers in the region B remain in a warmed up state. As a result, a population inversion required for laser oscillation typically cannot be achieved.
Accordingly, to perform laser oscillation in which the oscillation wavelength is selected at the terahertz band, the known quantum cascade laser must typically be cooled to a low temperature (95 K or less, Rüdeger Köhler et al., Applied Physics Letters, vol. 84 (2004), 1266). Therefore, a semiconductor laser device that performs laser oscillation at higher temperatures (for example, room temperature of 300K) is desired.